The present invention is about a next-generation hot mix asphalt (HMA) or warm mix asphalt (WMA) producer that is the main unit in the innovative next generation asphalt plant whose processing mechanism excels over any other existing asphalt plants available nowadays. It should be noted that principles of this invention apply both to HMA and WMA material. Therefore, for purposes of this Application, and unless otherwise noted, the term HMA should also be understood to include WMA. Likewise, the terminology RAP used hereafter also contains ARS.
The existing HMA plant has the following common units. Cold bins and belt conveyors supply virgin aggregates to the hopper of the mixing drum (or a producer). A mixing drum is the main unit that produces HMA product. A conventional burner consuming oil fuel generates the horizontal hot air stream along the mixing drum. The horizontal hot air stream that passes through cold aggregates circularly spraying down from the top toward the bottom, heats each cold aggregate. During this passage, the horizontal hot air stream picks up substantial amount of dust falling down with aggregates. Dust collector units facilitated at the end of the mixing drum eliminate most of dust before discharge of the hot air stream into the ambient air. Pocket flights attached on the inside of the mixing drum, initially transfer cold aggregates forward in the hot air stream for heating, by making aggregates circularly tumbled. The circular tumbling is caused by rotation of the inclined mixing drum. Heating of aggregates in this manner take place in the heating zone of the mixing drum, and then heated aggregates reach in the mixing zone. A RAP adding equipment inputs RAP at the mixing zone usually located at the almost end portion of the mixing drum, and, at the same time, a hot asphalt binder from asphalt storage tank are also sprayed into the same mixing zone. Heated virgin-aggregates and cold RAP-aggregates, and the hot asphalt binder meet at the mixing zone and mix together to produce HMA product at a given high temperature (usually 160° C.). A storage silo stores the well-mixed HMA transferred from the exit of the drum mixer by a belt conveyor, before loading into the dump truck that carries HMA on the construction site. Note that, at the mixing zone, the cold RAP begins to receive heat upon contacting both heated virgin aggregates and hot asphalt binders by exchanging heat each other, and the total mix gradually reaches the uniformly high mix temperature (160° C.). Thus, the amount of cold RAP input cannot exceed more than 50% of the total mix due to heat exchange requirement. The 30% RAP use is the common practice instead of 50% in the present RAP recycling industry. This is the one of the major limitations to be resolved immediately in the existing HMA plants.
The structure of existing asphalt mixing plants contains many technical limitations that need resolution. The following sentences explain those limitations. (1) Use of a general oil burner causes incomplete combustion that evolves air pollutant gases. (2) Simple mixing obtained by tumbling of materials in the mixing zone results to mediocre mix quality of HMA produced. (3) Addition of RAP in the mixing zone usually producing inferior HMA quality due to not enough heating time and heating energy that could make early damage of asphalt pavements (rutting, fatigue cracking, potholes, etc.). (4) Preventing dust generation is impossible under this process such that dust collectors are essential in removing dusts from the exhausting air. (5) Some fine dusts and blue smokes still escape from the dust collectors and contaminate the ambient air. (6) Limitation of the maximum RAP-recycling ratio is less than 50%, but usual practice is 15-30%.
Researchers have tried to remove limitation of the present maximum RAP recycling ratio of less than 50% and increase it up to 100% as claimed by U.S. Pat. Nos. 5,520,342, and 7,669,792. In U.S. and Europe, many companies have invested substantial money to develop the new HMA plant that is capable of 100% RAP recycling. Such HMA plants are reviewed by M. Zaumanis, R. B. Mallick & R. Frank in their work “100% Recycled Hot Mix Asphalt: A Review and Analysis,” (Elsevier, Resources, Conservation & Recycling, 92(2014), pp. 230-245). Most of these plants face on limitations listed in the following.
(1) Insufficient heating energy provided by the conventional oil burner or oil pipes fails to produce good quality of HMA. Heating energy is not sufficient due to parallel heating along the material passage and indirect heating of multiple particles (compared to direct heating of an individual particle in the existing plant) in spite of 3-times more heating energy required for 100% RAP heating than the regular virgin aggregate heating as shown in TABLE 1. Note in TABLE 1 that comparison of heating energy requirement among different materials is to compare the heat diffusivity among materials, because heat diffusivity represents the combined effect of the heat conductivity and the capacity. Since average heat diffusivity of granite aggregate is 3.28 times higher than the asphalt mix (or RAP), heating RAP requires 3.28 times more energy required compared to the heating virgin aggregates to get the same temperature. This means that the heating energy of the oil burner used in the 100% RAP recycling plant should be 3.28 times higher than heating energy of the virgin aggregates. Also the parallel heating along the material passage in all those 100% RAP recycling plants provides a lot less efficient compared to the perpendicular heating on materials. Also the indirect heating of multiple RAP particles requires more energy compared to the direct heating of an individual particle in the existing plant. All these heating factors negatively affect the success of the 100% RAP recycling plants developed so far. Use of the higher heating energy burner and design of the efficient heating system are prerequisite for successful development of the 100% RAP recycling process.
TABLE 1HeatRef.ConductivityHeat CapacityDensityHeat DiffusivityMaterialsNok (W/m/°K)Cp (KJ/Kg/°K)ρ (Kg/m3)α × 106 (m2/s)RAP or10.75Av = 0.920Av =2300Av = 23200.35Av =Asphalt20.7-1.41.051.270.992300(1)0.360.46Mixes(1.05)(1)(1)(1)30.8-1.060.85-0.8724000.37-0.53(0.93)(0.86)(0.45)41.210.9223000.5751.21-1.380.84-1.0923000.58(1.30)(0.97)23000.58Granite11.7-4.0Av =0.79Av =2350Av =1.535Av =Aggregate(2.85)2.830.802350 (1.01)1.516 2.813(2.7) 0.816(0.8)23501.467(3.28)Carbon143   Av = 0.466Av =7873Av =11.72Av =Steel745-6850.00.44-0.5 0.4778307852 (3.38)18.014.86(56.5) (47.6)(0.47)(32.3)1. www.engineeringtoolbox.com, “Thermal Conductivity of Some Common Materials and Gasses.”2. M. S. Mamlouk, MW Witczak, K. E. Kaloush, & N Hasan, “Determination of Thermal Properties of Asphalt Mixes,” ASTM (International), Vol. 33, Issue 2, March 2005.3. P. G. Jordan & M. E. Thomas, “Predictions of Cooling Curves for Hot-Mix Paving Materials by a Computer Program,” Transport and Road Research Laboratory Report 729, 1976.4. J. S. Corlew & P. F. Dickson, “Methods for Calculating Temperature Profiles of Hot-Mix Asphalt Concrete as related to the Construction of Asphalt Pavements,” Asphalt Paving Technology 1968, Proceedings of Association of Asphalt Paving Technologists Technical Sessions, Vol. 37, pp. 101-140.5. P. A. Tegeler & B. J. Dempsey, “A Method of Predicting Compaction Time for Hot-Mix Bituminous Concrete,” Asphalt Paving Technology 1973, Proceedings of Association of Asphalt Paving Technologists Technical Sessions, Vol. 42, pp. 499-523.6. J Kim, Y Lee & M Koo, “Thermal Properties of Granite from Korea,” American Geophysical Union, Fall Meeting 2007, Abstract #T11B-0576.7. M. Sedighi & B. N. Dardashti, “A Review of Thermal & Mechanical Analysis in Single & Bi-Layer Plates,” J of Materials Physics & Mechanics, Vol. 14, PP. 37-46, 2012.
(2) Poor mixing could be another problem of those 100% RAP recycling plants because mixing only relies on blending of tumbled materials caused by the rotation of the inclined mixing drum, without any frictional shearing. Frictional shear mixing can cause the convectional heating to materials instead of the conductive heating in the conventional tumbling mixing. Poor mixing requires additional mixing tool like a pug mill in those 100% RAP recycling plants, but this mixing usually performs at an ambient temperature without heating. The poor heating and mixing that cannot be solved are big obstacles in getting decent HMA production.
(3) A conventional oil burners or oil pipes used in those 100% RAP recycling plants make 80% combustion of fuel, and thus produce more air pollutants like NOx, SOx, CO, CO2, etc. compared to 100% combustion.
(4) Manufacturing and installation cost of those 100% RAP recycling plants are usually too high to be practical due to complicated equipment needed.
(5) These plants can produce only small amount of good quality 100% RAP-recycled mixes that is not practical in reality.
(6) The only innovation made from the existing HMA plants is the complete separation of material flow from heat transfer to make indirect heating necessary for 100% RAP recycling and no dust collector facilitation. Unfortunately, most of those plants are idle or scarcely used in operation due to limitations explained above.